How can I save a dandelion for ever. Underneath some type of capsule
What is your project? Dandelions are everywhere and no shortage of them. You could embed it resin-like a paper weight.
Put it in resin.
In resin. Go to your craft store. Get a mold and some resin. Put your flower in the mold, mix the resin, por it into the mold completely covering the flower, then let it sit until the resin hardens. Finally pop it out of the mold. You could also press the flower. Then you wouldn't need such a deep mold.
You can put it in between 2 sheets of wax paper. Then iron the sheets of wax paper to enclose the flower. The heat from the iron will melt the wax so that you have it forever. After you iron the sheets together, you can cut the wax paper into any shape you want.